Si@SiOx@C Layer-by-Layer Superstructure with

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MXene/Si@SiOx@C Layer-by-Layer Superstructure with AutoAdjustable Function for Superior Stable Lithium Storage Yelong Zhang, Zijie Mu, Jianping Lai, Yuguang Chao, Yong Yang, Peng Zhou, Yiju Li, Wenxiu Yang, Zhonghong Xia, and Shaojun Guo ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b08821 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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MXene/Si@SiOx@C Layer-by-Layer Superstructure with Auto-Adjustable Function for Superior Stable Lithium Storage Yelong Zhang1, 2, Zijie Mu1, Jianping Lai1, Yuguang Chao1, Yong Yang1, Peng Zhou1, Yiju Li1, Wenxiu Yang1, Zhonghong Xia1 and Shaojun Guo1, 3* 1

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China.

2

School of Applied Physics and Materials, Wuyi University, 22 Dongcheng Village, Jiangmen 529020, P. R. China. 3

BIC-ESAT, College of Engineering, Peking University, Beijing 100871, P. R. China. *

To whom correspondence should be addressed. E-mail: [email protected]

ABSTRACT: Despite its very high capacity (4200 mAh g-1), the widespread application of the silicon anode is still hampered by severe volume changes (up to 300%) during cycling, which results in electrical contact loss and thus dramatic capacity fading with poor cycle life. To address this challenge, 3D advanced Mxene/Si-based superstructure including MXene matrix, silicon, SiOx layer and nitrogen-doped carbon (MXene/Si@SiOx@C) in a layer-by-layer manner were rationally designed and fabricated for boosting lithium-ion batteries (LIBs). The MXene/Si@SiOx@C anode takes the advantages of high Li+ ion capacity offered by Si, mechanical stability by the synergistic effect of SiOx, MXene, and N-doped carbon coating and excellent structural stability by forming a strong Ti-N bond among the layers. Such an interesting superstructure boosts the lithium storage performance (390 mAh g-1 with 99.9% Coulombic efficiency and 76.4% capacity retention after 1000 cycles at 10 C) and effectively suppresses electrode swelling only to 12% with no noticeable fracture or pulverization after long-term cycling. Furthermore, a soft package full LIB with MXene/Si@SiOx@C anode and Li[Ni0.6Co0.2Mn0.2]O2 (NCM622) cathode was demonstrated, which delivers a stable capacity of 171 mAh g-1 at 0.2 C, a promising energy density of 485 Wh kg-1 based

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on positive active material, as well as good cycling stability for 200 cycles even after bending. The present MXene/Si@SiOx@C becomes among the best Si-based anode materials for LIBs. KEYWORDS: silicon anodes, MXene, layer-by-layer superstructure, lithium-ion batteries

Driven by the rapidly expanding use of portable electronics, power tools, electric and hybrid vehicles, the development of rechargeable lithium-ion batteries (LIBs) with high capacity, long cycle life and low cost has become a worldwide priority.1 Silicon is considered to be strong candidates of anode because of their ultrahigh theoretical capacity of 4200 mAh g-1, natural abundance, and relatively low Li-uptake voltage.2 However, two intrinsic obstacles, usually inducing severe capacity decay, extremely restrict its practical application: (a) the huge volume expansions and contractions over repeated lithiation/delithiation cycles, causing Si particles pulverization and the formation of an unstable solid electrolyte interphase (SEI), which result in continuous consumption of electrolyte as well as drastic capacity fading;3 (b) low electrical and ionic conductivity often impair rate capability of Si-based materials.4 Therefore, major efforts on Si anode have been devoted to how to better accommodate the large strains without obvious volume variations,5 producing a stable SEI film and enhancing their electronic and ionic conductivity.6,7 By partially addressing all aforementioned issues of Si anode during the insertion/extraction process, Si@SiOx core/shell nanostructures have been considered as one of the most promising alternatives for LIBs mainly due to their relative good cycling stability.8 After insertion of lithium into SiOx, lithium oxide or/and lithium silicates are irreversibly produced, leading to buffer matrixes to accommodate an acceptable degree of volume expansions and promote the formation of a stable SEI layer.9 Unfortunately, many obstacles are still needed to be addressed, like poor rate capability and extremely limited cycling life at high rates owing to intrinsically low electrical conductivity and poor Li diffusivity.4 Newly 2D transition metal carbides termed as MXenes are of great interest because of their excellent electrical, electrochemical, and mechanical behaviors and promising applications in batteries and catalysis.10 Ti3C2 MXene has shown the obvious advantage in transparent, flexible supercapacitors,11 microsupercapacitors and Li-S batteries.12,13 In particular, the large interlayer distance of MXenes may allow the Si-based materials be intercalated flexibly, providing elastic


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buffer to accommodate the volume variation. Inspired by the 2D flexible layer and 3D stacking structure of Ti3C2 MXene with metallic conductivity, we report a class of MXene/Si@SiOx@C layer-by-layer assembling superstructure as anode for achieving high-performance LIBs applications with superior stability, high capacity and rate ability. This superstructure was fabricated through intergrating the Stöber method, magnesiothermic reduction and carbonation. Totally different from previously reported silicon/carbon composites, the MXene/Si@SiOx@C have three distinguishing features for promoting the performance of LIBs: (i) MXene acts as the 3D matrix to enhance the electronic conductivity and shorten the Li+ ions diffusion pathway. (ii) Si particles anchored onto the layer-stacked MXene surface will efficiently accommodate strain induced by volume changes during the electrochemical reaction processes. (iii) The synergistic effects from MXene, SiOx, and N-doped carbon coating layer endow the assembling superstructure with stable mechanical properties and physical supports to protect Si that is vulnerable to fracture. These important advantages make MXene/Si@SiOx@C anode display extraordinary structural stability (only 12% swelling rate after 1000 cycles at 10 C) with superior reversible capacity, high Coulombic efficiency, excellent cycling stability and rate performance, as well as reliable operation in a soft package full LIB.

RESULTS AND DISCUSSION An Figure 1a depicts the schematic diagram for making MXene/Si@SiOx@C layer-by-layer superstructure. First, layered Ti3C2Tx MXenes were made by removing Al-layers from Ti3AlC2 MAX phases by hydrofluoric acid (HF) etching.14 Then, the MXene/SiO2 nanoparticles were synthesized by growing SiO2 partials on the MXene surface using the Stöber method. After an in situ magnesiothermic-reduction procedure for converting the MXene/SiO2 to MXene/Si, direct pyrolysis of poly (methyl methacrylate) (PMMA) polymer on the surface of Si nanoparticles using urea as nitrogen source was conducted for making the MXene/Si@SiOx@C. During the pyrolysis process, the ester group in PMMA can be chemically bound on the surface of MXene/Si and thermally decomposed to carbon with the simultaneous formation of SiOx.15 In addition, the contents of Si in the resulting nanohybrid could be easily adjusted by controlling the hydrolysis time of Si tetraethoxide (TEOS). Thus, 3D MXene/Si@SiOx@C nanohybrids with the different Si contents of 64.8 wt%, 72.8 wt% and 78.7 wt% could be prepared, denoted as MXene/Si@SiOx@C-1, MXene/Si@SiOx@C-2, and MXene/Si@SiOx@C-3, respectively (Table 1).


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The morphologies of as-made products were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The micrometer-sized Ti3C2Tx MXene exhibits obvious open lamellar characteristic (Figure 1b, S1 and S2). The Si nanoparticles were uniformly anchored onto the surface of the MXene nanosheet after the magnesiothermic reduction of MXene/SiOx (Figure 1c). By coating PMMA and further carbonization with urea, thin SiO x and carbon layer were encapsulated on the surface of MXene/Si (Figure 1d, e), mostly like to form a 3D multiporous and sandwich-like structure. High resolution TEM (HRTEM) image of the MXene/Si@SiOx@C further proves a compact carbon is coated on the surface of SiOx nanoparticle (Figure 1f). The good crystallinity nature of Si is confirmed by the measured interplanar distances of 0.31 nm, matching well with the Si(111) crystallographic plane (Figure 1g).16 The energy dispersive X-ray (EDX) elemental mappings (Figure 1h) clearly reveal that Si, Ti, C, O and N are homogeneously distributed in the MXene/Si@SiOx@C. To further elucidate the structure information and surface properties, the X-ray diffraction (XRD), Raman spectroscopy, Brunauer-Emmett-Telley (BET) method and X-ray photoelectron spectroscopy (XPS) were performed. Two strong diffraction peaks appear at 28.4o and 47.3o in the XRD pattern of MXene/Si@SiOx@C, indexed to (111) and (220) diffraction peaks of cubic Si with a diamond structure (JCPDS no.27-1402), respectively (Figure 2a).17 The obvious peaks located in the vicinity of 7.9° and 39.3° are observed in MXene/Si@SiOx@C, due to the characteristic reﬂections of MXene phase.18 Interestingly, the main peak (002) of MXene is shifted toward lower angles after compositing with Si@SiOx@C (Figure 2b), suggesting an increased MXene layer spacing along the c-axis and also its interesting flexible characteristic. The expanded layer spacing of MXene endows a relatively lower Li+ ions diffusion resistance and larger interspace for accommodating volume change of Si over cycling.19 The presence of the carbon coatings on the surface of the MXene/Si@SiOx@C is confirmed by Raman spectra (Figure 2c). The two broad peaks observed at around 1350 and 1590 cm-1 are assigned to the defect-induced (D) mode and stretching mode of C-C bonds (G) of graphite, respectively.20 Furthermore, the values of the ratio ID/IG for MXene/Si@SiOx@C samples (in the range of 1.09-1.21) are significantly higher than that for the commercial Si/C (0.72), implying that plentiful defects and vacancies were generated during the synthesis process (carbonation of PMMA and urea).21 The specific surface areas of MXene/Si@SiOx@C-1, MXene/Si@SiOx@C-2 and


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MXene/Si@SiOx@C-3 are calculated to be 167.2, 224.3 and 172.5 m2 g-1 based on the Brunauer-Emmett-Teller analysis (Figure 2d), much larger than those of pure MXene (37.2 m2 g-1), respectively. The lower specific surface area of MXene/Si@SiOx@C-3 than that of MXene/Si@SiOx@C-2 indicates that excessive amounts of Si may block the diffusion channel to a certain degree. The larger specific surface area of MXene/Si@SiOx@C-2 can better facilitate the penetration of electrolyte and shorten the length of Li+ ions diffusion path.22 XPS was used to further investigate the chemical state of MXene/Si@SiOx@C. The measured band of the Si 2p binding energy is divided into five sub-bands (Figure 2e): SiO2.0 (103.5 eV), SiO1.5 (102.7 eV), SiO1.0 (101.5 eV), SiO0.5 (100.6 eV), and Si (98.7 eV).9 The calculated X value in MXene/Si@SiOx@C is 1.41 within the XPS probing depth based on the fitting results. Similarly, the N 1s high-resolution XPS spectrum is resolved into three single-peaks at 397.8, 399.6, and 400.5 eV (Figure 2f), corresponding to the pyridinic, pyrrolic, and graphitic types of nitrogen, respectively. 23 Moreover, the N 1s component centered at 396.4 eV can be assigned to the N-Ti bond, in consistent with the Ti-N peak centered at 455.0 eV in the Ti 2p spectra (Figure 2g).24 These XPS results also reveal that the N dopants are incorporated not only in the carbon layer but also in the planar layers of the MXene. Thus, constrained by the N-Ti bonds, in addition to pyridinic, pyrrolic, and graphitic types of nitrogen, the stability of the combination interface and 3D sandwich-like structure can be greatly enhanced by strong covalent bonds, which are critical in contributing to the improved cycling life. The Li storage performance of the as-prepared MXene/Si@SiOx@C nanohybrids was investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatically charge-discharge test using coin-type half-cells with the Li-metal as counter electrode. The Li+ ions intercalation/deintercalation phenomenon of MXene/Si@SiOx@C-2, commercial Si/C and bare MXene electrodes was first depicted by their initial CV curves in the voltage range of 0.01-2.5 V at 0.1 mV s-1 (Figure 3a). The main reduction peak in the range of 0.01-0.40 V was attributed to the formation of a Li-Si alloy during the discharge processes.25 The charging branch shows two oxidation peaks near 0.40 and 0.53 V, mainly ascribed to the extraction of Li + ions from LixSi.9 Compared with the bare MXene and commercial Si/C electrode, MXene/Si@SiOx@C-2 shows the decreased polarization between the cathodic and anodic peaks and increased peaks intensity, indicating its accelerated lithiation/delithiation kinetics.26 The EIS of MXene/Si@SiOx@C-1,


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MXene/Si@SiOx@C-2, MXene/Si@SiOx@C-3 and bare Si anodes structured half-cells are compared. The calculated Rct extracted from the high-frequency range were 22.1 and 24.7 Ω for the MXene/Si@SiOx@C-2 before (Figure 3b) and after (Figure S3a and Figure S3b) cycle testing, respectively, which are smaller than those MXene/Si@SiOx@C-1 (32.3 and 52.9 Ω), MXene/Si@SiOx@C-3 (44.2 and 81.7 Ω) and commercial Si/C (106.3 and 173.8 Ω). The EIS results of MXene/Si@SiOx@C-2 further demonstrate its lowest charge-transfer impedance, fastest electron transport, high stability as well as excellent reversibility.27 Figure 3c shows the galvanostatic discharge-charge profiles of the MXene/Si@SiOx@C-2 in the 0.01-2.5 V range for the 1st, 100th, and 200th cycles at 0.2 C. The MXene/Si@SiOx@C-2 electrode delivers a large reversible specific capacity of 1674 mAh g-1 at 0.2 C with an initial Coulombic efficiency of 81.3%. The cyclic performance comparison of different MXene/Si@SiOx@C was given in Figure 3d at current density of 0.2 C. The MXene/Si@SiOx@C-2 anode with a Coulombic efficiency of approximately 100% delivers a stable capacity of 1547 mAh g-1 after 200 cycles, much larger than those of MXene/Si@SiOx@C-1 (1049 mAh g-1), MXene/Si@SiOx@C-3 (1226 mAh g-1) and Si (238 mAh g-1), suggesting its super stable Li storage performance. The high rate capability is another

key requirement

for

practical

applications.

The

reversible

capacities

of

the

MXene/Si@SiOx@C-2-based battery are 1444, 1192, 810, 554 and 398 mAh g-1 at 0.5, 1, 2, 5 and 10 C, respectively (Figure 3e). When the current density is tuned back to 0.5 C, a specific capacity is recovered up to 1440 mAh g-1 (Figure 3f), corroborating its excellent structural stability during the repeated lithiation/delithiation processes. To further evaluate the long-term high-power performance, the cycle retention with Coulombic efficiency of MXene/Si@SiOx@C-2 anode at the ultrahigh rate of 10 C was carried out (Figure 3g). The MXene/Si@SiOx@C-2 anode achieves a reversible capacity of 510 mAh g-1 for the first cycle, and still retains the capacity of 390 mAh g-1 with 99.9% Coulombic efficiency after 1000 cycles, demonstrating a 76.4% capacity retention. The charge-transfer resistance obtained from EIS of the cycled battery is close to the value before cycling, demonstrating high stability as well as the excellent reversibility of MXene/Si@SiOx@C-2 electrode in long cycles test at high rates (Figure S4).28 To the best of our knowledge, the outstanding rate and cycle performance of MXene/Si@SiOx@C-2 are among the best in all the reported Si-based anode materials (Table S1).


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Ex-situ XRD analysis was carried out to investigate the buffering function of MXene matrix. Since the interlayer spacing is an important parameter to evaluate the elasticity of MXene layers, the MXene (00l) XRD peaks in the 2θ angle range of 5-10o for MXene/Si@SiOx@C electrode were recorded at various potentials (Figure 4a). In the discharge process, a progressive and continuous shift of the MXene (00l) peaks to lower values (from 7.3o to 6.4 o) is observed with decreased discharge potential from 2.5 to 0.01 V, corresponding to a continuous increase of interlayer spacing (from 1.31 to 1.53 nm).27 Then the peaks reversibly shift back to their original position when charge from 0.01 V back to 2.5 V, indicating the decrease of interlayer spacing (from 1.53 back to 1.30 nm). Comparing the interlayer spacing data in discharge and charge process, the promising elastic characteristics of MXene matrix were observed, suggesting high adjustability and reversibility. Overall, the buffering and protective effects of MXene with auto-adjustable interlayer spacing can be described as following: in the discharge process, the lithiation of Si-based active material led to an increase of the interlayer spacing. In opposite, the delithiation accounts for the observed decrease of the interlayer spacing in the charge process. In order to deeply elucidate the positive effect of auto-adjustable MXene matrix in high-rate and long-term cycling, the morphological and structural changes of MXene/Si@SiOx@C electrode were examined. Apparently, MXene/Si@SiOx@C electrode after 1000 cycles at 10 C still retains its original appearance (Figure 4b, S5a and S5c). In contrast, the commercial Si/C electrode generates large cracks and presents a peel-off phenomenon (Figure 4c, S5b and Figure S5d) due to the huge volume expansions of Si nanoparticles. The thickness of the cycled MXene/Si@SiOx@C electrode was only increased from 25 to 28 μm with a small swelling rate of 12% (Figure 4d, e). However, the Si/C electrode exhibits a large swelling rate of 53.5% (Figure S6a, S6b), thus leading to the large electrical contact loss and rapid performance fading.29 The accordion-like MXene morphology of MXene/Si@SiOx@C could be well preserved after high-rate and long-term cycling (Figure 4f, 4g). The outstanding stability and reversibility of MXene/Si@SiOx@C electrode can be attributed to the auto-adjustable MXene matrix acting as the confining buffer to efficiently cushion the massive volume changes upon cycling.30 Considering the requirements for a variety of electronic devices, the excellent performance of MXene/Si@SiOx@C-2 half-cell encourages us to further evaluate its practicability in a commercial viable prototype. Li[Ni0.6Co0.2Mn0.2]O2(NCM 622) as one of the Ni-rich layered cathode materials is


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interesting due to its substantial energy capacity, wide operating voltage window and low cost. 31 The initial discharge capacity of a NCM622-based coin-type half-cell with a Li metal anode reaches 187 mAh g-1 (Figure S7a), and the discharge capacity retention is found to be 98.5% after 200 cycles at 0.2 C in the voltage range 3.0-4.3 V (Figure S7b). Afterward, the MXene/Si@SiOx@C-2 anode was paired with a Li[Ni0.6Co0.2Mn0.2]O2 cathode in an Al-plastic film soft package full battery. The corresponding redox reaction is based on the following equation (Figure 5a): Li[Ni0.6Co0.2Mn0.2]O2 + [MXene/Si@SiOx@C] = Li1-x[Ni0.6Co0.2Mn0.2]O2 + Lix[MXene/Si@SiOx@C]

(1)

Specially, the NCM622 electrode in a soft package full battery delivers high capacities of 181, 178, 175, 173 and 171 mAh g-1 at 1st, 50th, 100th, 150th and 200th (Figure 5b), respectively, which is highly comparable to a NCM622-based coin-type half-cell. The full-cell demonstrates a maximum energy density of 485 Wh kg-1 (calculated based on the NCM 622 mass by integrating the discharge curve), which is considerably larger than that obtained from the state-of-the-art Li-ion batteries (about 170 Wh kg-1). Also, a light-emitting diode (LED) can be successfully powered by a soft package full battery (Inset in Figure 5b), driving for at least 150 min (Figure 5c). More importantly, Figure 5d displays a stable cyclability over 200 cycles with a high Coulombic efficiency near 100% even when bended to different angles. All these results strongly reveal MXene/Si@SiOx@C-2 have potential for practical applications. The outstanding performance should be contributed to the following reasons: (1) the porous 3D networks are beneﬁcial for rapid Li+ ions diffusion and electrolyte soaking; (2) high electrical conductivity of Ti3C2 MXene and N-doped carbon provides highly efficient conductive substrate in not only the nanoscale but also mesoscale levels, leading to better electrical transport capabilities and faster electrode kinetics; (3) the synergistic effect of MXene with variable layer space, SiOx and N-doped carbon coating will efficiently accommodate strain induced by volume changes of Si during the electrochemical reaction processes; and (4) last but not the least, the N-Ti bonds along with pyridinic, pyrrolic, and graphitic types of nitrogen can provide strong anchoring conditions to prevent large cracks and undesirable peel-off phenomenon. These will undoubtedly contribute to the enhancement of Li storage performance.

CONCLUSIONS


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To summarize, we have successfully demonstrated a class of MXene/Si@SiOx@C layer-by-layer superstructure with auto-adjustable layer space for boosting the LIB performance in terms of stability, rate ability and capacity. Rationally designed MXene/Si@SiOx@C layer-by-layer superstructure exhibits the merits of increased active surface area, favorable electronic conductivity, rapid Li + mobility, flexible layer spacing and rich covalent bonds, which are ideal materials for accommodating the large volume changes of Si during cycling. With these extraordinary characteristics, the MXene/Si@SiOx@C nanohybrids exhibit outstanding lithium storage properties, including higher capacity, superior rate capability and better cycling stability. When paired with a NCM622 cathode, the soft pack full-cell delivers high energy density of 485 Wh kg-1 as well as good cycling stability for 200 cycles even after bending with various shapes. As a result, the rational design together with superior storage performance will rapidly accelerate the construction of stable silicon anodes with improved performance for next generation LIBs.


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EXPERIMENTAL METHODS Preparation of Ti3AlC2 MAX. The Ti3AlC2 MAX was synthesized by mixing Ti2AlC (≥ 92 %, 3-ONE-2) and TiC (≥ 99%, Sigma-Aldrich) powders with a 1:1 molar ratio via a simple dry ball milling for 22 h. The mixture powders were heated in a tube furnace at 1350 °C for 2 h in Ar ambient. The resulting lightly sintered blocks were milled with a titanium-nitride-coated milling bit. After being sieved using a stainless steel 300-mesh sieve, the Ti3AlC2 samples were obtained and used for further study. Synthesis of Ti3C2 MXene. MXene was prepared by the exfoliation of Ti3AlC2. The resulting Ti3AlC2 (2 g) sample was treated with aqueous 48% HF solution (10 mL, Sigma-Aldrich) at 60 oC for 4 h. The resultant suspension was then washed using distilled water three times (15 min each wash) and then filtered to obtain Ti3C2Tx MXene. Synthesis of MXene/Si@SiOx@C Nanohybrids. 200 mg of MXene was immerged into 4 mL mixed aqueous ethanol solution of TEOS (volume ratio of water: alcohol is 70:30) for 4 h. Magnesium vapor was then used to reduce silica to Si at 650 °C under a 4% H2/Ar atmosphere with a flow rate of 0.2 L min-1. Subsequently, excess Mg was removed by the diluted HCl solution. Finally, MXene/Si@SiOx@C-2 nanohybrids with Si contents of 74.3 wt% were obtained (see Table S2 for the details). For the comparison, the contents of Si in the resulting nanohybrid could be easily adjusted by controlling the hydrolysis time of TEOS. Thus, 3D MXene/Si@SiO x@C nanohybrids with the different Si contents of 64.8 wt% (TEOS hydrolysis time: 3 h) and 78.7 wt% (TEOS hydrolysis

time:

5

h)

could

be

prepared,

denoted

as

MXene/Si@SiOx@C-1

an

MXene/Si@SiOx@C-3, respectively. Commercial nano-Si powder (about 50 nm, 98%, Alfa) was also employed for comparison. The Si contents in 3D MXene/Si@SiOx@C nanohybrids were obtained by XPS (Table 1). To further check the accuracy and validity of XPS results, all the MXene/Si@SiOx@C samples were stirred in aqueous 48% HF solution for 24 h to remove Si/SiOx. 66.8 wt%, 75.1 wt % and 81.5 wt % mass reduction were observed for MXene/Si@SiOx@C-1, MXene/Si@SiOx@C-2 and MXene/Si@SiOx@C-3, respectively, agreeing reasonably well with the XPS results. Structural Characterization. The morphologies and microstructures were obtained using field emission scanning electron microscopy (FESEM; JEOL JSM-7000F) and high-resolution transmission electron microscopy (HRTEM; JEOL 2100F). Powder X-ray diffraction (XRD) patterns were obtained using an X-ray diffractometer (Empyrean, PAN analytical) with a 3D pixel semiconductor detector using Cu Kα radiation. The BET results were carried out by nitrogen


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sorption with ASAP 2010 at 77 K and analyzed based on Brunauer-Emmett-Teller theory. Raman spectra were performed on a Raman spectrometer (Bruker Senterra Grating 400) at 532 nm employing a He-Ne lase. X-ray photoelectron spectroscopies (XPS) were acquired using Thermo Scientific K-Alpha spectrometer. Electrochemical Measurements. The working electrodes were prepared by mixing active materials, conducting

agent

(acetylene

black)

and

PVDF

in

a

weight

ratio

of

80:10:10

in

N-methyl-2-pyrrolidone (NMP) to form the slurry. After coating on current collector, the electrode was dried at 80 °C for 12 h. The CR2032 coin cells were used to evaluate electrochemical performance of half-cells. A polyethylene (PE) microporous membrane was served as a separator, and the electrolyte was the mixture of ethylene carbonate (EC) and propylene carbonate (PC) with a volume ratio of 1:1 containing 1 M LiPF6. The voltage range was 0.01-2.5V vs Li/Li+ for galvanostatically charge and discharge tests of half-cells at room temperature. Impedance Nyquist plots were obtained in the frequency range of 100 kHz to 0.01 Hz at the open circuit potential. The capacity of half-cell is based on the Si mass. The mass loading of Si active materials was controlled to be ~1 mg cm-2geo based on the XPS results. For the construction of soft package full cell, the same electrolyte and separator were used. Li[Ni0.6Co0.2Mn0.2]O2 (NCM622, Changchun Institute of Applied Chemistry) mixed with acetylene black and PVDF (80:10:10, in mass) was spread on Al foil and employed as the cathode. The mass ratio of MXene/Si@SiOx@C-2 to NCM622 was controlled at about 1.5: 1 to balance the capacity. The soft package full battery was first activated at a relatively low current density of 0.05 C for 3 cycles, and then charged/discharged (current density based on the mass of NCM622). All of the electrochemical experiments were carried out at room temperature. ASSOCIATED CONTENT Supporting Information Available This material is available free of charge via the Internet at http://pubs.acs.org. More characterizations of MXene/Si@SiOx@C, including Figures S1−S7 and Tables S1;

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]

ACKNOWLEDGMENTS


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This work was financially supported by National Key R&D Program of China (No. 2016YFB0100201), Beijing Natural Science Foundation (JQ18005), National Natural Science Foundation of China (51671003), BIC-ESAT funding, Young Thousand Talented Program, China Postdoctoral Science Foundation (2017M620521), the Science Foundation for High-Level Talents of Wuyi University (2018RC50 and 2017RC23), and the Department of Education Guangdong Province (2017KCXTD031).


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REFERENCES 1. Armand, M.; Tarascon, J. M., Building Better Batteries. Nature 2008, 451, 652-657. 2. Zuo, X.; Zhu, J.; Mullerbuschbaum, P.; Cheng, Y., Silicon Based Lithium-Ion Battery Anodes: A Chronicle Perspective Review. Nano Energy 2017, 31, 113-143. 3. Yang, J.; Li, S.; Kushima, A.; Zheng, X.; Sun, Y.; Jin, X.; Jie, S.; Xue, W.; Zhou, G.; Jiang, W., Self-healing SEI Enables Full-Cell Cycling of a Silicon-Majority Anode with a Coulombic Efficiency Exceeding 99.9%. Energy Environ. Sci. 2017, 10, 580-592. 4. Jin, Y.; Zhu, B.; Lu, Z.; Liu, N.; Zhu, J., Challenges and Recent Progress in the Development of Si Anodes for Lithium-Ion Battery. Adv. Energy Mater. 2017, 7, 1700715. 5. Xu, Q.; Li, J. Y.; Sun, J. K.; Yin, Y. X.; Wan, L. J.; Guo, Y. G., Watermelon-Inspired Si/C Microspheres with Hierarchical Buffer Structures for Densely Compacted Lithium-Ion Battery Anodes. Adv. Energy Mater. 2016, 7, 1601481. 6. Kim, N.; Chae, S.; Ma, J.; Ko, M.; Cho, J., Fast-Charging High-energy Lithium-Ion Batteries via Implantation of Amorphous Silicon Nanolayer in Edge-Plane Activated Graphite Anodes. Nat. Commun. 2017, 8, 812. 7. Kwon, T. W.; Choi, J. W.; Coskun, A., The Emerging Era of Supramolecular Polymeric Binders in Silicon Anodes. Chem. Soc. Rev. 2018, 47, 2145-2164. 8. Lee, S. J.; Kim, H. J.; Hwang, T. H.; Choi, S.; Park, S. H.; Deniz, E.; Jung, D. S.; Choi, J. W., Delicate Structural Control of Si-SiOx-C Composite via High-Speed Spray Pyrolysis for Li-Ion Battery Anodes. Nano Lett. 2017, 17, 1870-1876. 9. Yang, C.; Zhang, Y.; Zhou, J.; Lin, C.; Lv, F.; Wang, K.; Feng, J.; Xu, Z.; Li, J.; Guo, S., Hollow Si/SiOx Nanosphere/Nitrogen-Doped Carbon Superstructure with a Double Shell and Void for High-Rate and Long-Life Lithium-Ion Storage. J. Mater. Chem. A 2018, 6, 8039-8046. 10. Kim, S. J.; Koh, H.; Ren, C. E.; Kwon, O.; Maleski, K.; Cho, S.; Anasori, B.; Kim, C.; Choi, Y.; Kim, J., Metallic Ti3C2Tx MXene Gas Sensors with Ultrahigh Signal-to-Noise Ratio. ACS Nano 2018, 12 , 986-993. 11. Zhang, C. J.; Anasori, B.; Seralascaso, A.; Park, S.; Mcevoy, N.; Shmeliov, A.; Duesberg, G. S.; Coleman, J. N.; Gogotsi, Y.; Nicolosi, V., Transparent, Flexible, and Conductive 2D Titanium Carbide (MXene) Films with High Volumetric Capacitance. Adv. Mater. 2017, 29, 1702678. 12. Zhang, C. J.; Kremer, M. P.; Seralascaso, A.; Park, S.; Mcevoy, N.; Anasori, B.; Gogotsi, Y.; Nicolosi, V., Stamping of Flexible, Coplanar Micro-Supercapacitors Using MXene Inks. Adv. Funct. Mater. 2018, 28, 1705506. 13. Tang, H.; Li, W.; Pan, L.; Cullen, C. P.; Liu, Y.; Pakdel, A.; Long, D.; Yang, J.; Mcevoy, N.; Duesberg, G. S., In Situ Formed Protective Barrier Enabled by Sulfur@Titanium Carbide (MXene) Ink for Achieving High-Capacity, Long Lifetime Li-S Batteries. Adv. Sci. 2018, 5, 1800502. 14. Zhang, Y.; Mu, Z.; Yang, C.; Xu, Z.; Zhang, S.; Zhang, X.; Li, Y.; Lai, J.; Sun, Z.; Yang, Y., Rational Design of MXene/1T-2H MoS2-C Nanohybrids for High-Performance Lithium-Sulfur Batteries. Adv. Funct. Mater. 2018, 1707578. 15. Jiang, B.; Zeng, S.; Wang, H.; Liu, D.; Qian, J.; Cao, Y.; Yang, H.; Ai, X. P., Dual Core-Shell Structured Si@SiOx@C Nanocomposite Synthesized via a One-Step Pyrolysis Method as a Highly Stable Anode Material for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2016, 8, 31611-31616. 16. Rehman, S.; Guo, S.; Hou, Y., Rational Design of Si/SiO2@Hierarchical Porous Carbon Spheres as Efficient Polysulfide Reservoirs for High-Performance Li-S Battery. Adv. Mater. 2016, 28, 3167-3172.


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17. Liang, J.; Li, X.; Hou, Z.; Guo, C.; Zhu, Y.; Qian, Y., Nanoporous Silicon Prepared through Air-Oxidation Demagnesiation of Mg2Si and Properties of Its Lithium Ion Batteries. Chem. Commun. 2015, 51, 7230-7233. 18. Liang, X.; Rangom, Y.; Kwok, C. Y.; Pang, Q.; Nazar, L. F., Interwoven MXene Nanosheet/Carbon-Nanotube Composites as Li-S Cathode Hosts. Adv. Mater. 2017, 29, 1603040. 19. Lian, P.; Dong, Y.; Wu, Z. S.; Zheng, S.; Wang, X.; Wang, S.; Sun, C.; Qin, J.; Shi, X.; Bao, X., Alkalized Ti3C2 MXene Nanoribbons with Expanded Interlayer Spacing for High-Capacity Sodium and Potassium Ion Batteries. Nano Energy 2017, 40, 1-8. 20. Geng, X.; Zhang, Y.; Jiao, L.; Yang, L.; Hamel, J.; Giummarella, N.; Henriksson, G.; Zhang, L.; Zhu, H., Bioinspired Ultrastable Lignin Cathode via Graphene Reconfiguration for Energy Storage. ACS Sustainable Chem. Eng. 2017, 5, 3553-3561. 21. Zhu, J.; Xiao, M.; Zhang, Y.; Jin, Z.; Peng, Z.; Liu, C.; Chen, S.; Ge, J.; Xing, W., Metal-Organic Framework-Induced Synthesis of Ultrasmall Encased NiFe Nanoparticles Coupling with Graphene as an Efficient Oxygen Electrode for a Rechargeable Zn-Air Battery. ACS Catal. 2016, 6, 6335-6342. 22. Zhang, Y.; Cui, Q.; Zhang, X.; Mckee, W. C.; Xu, Y.; Ling, S.; Li, H.; Zhong, G.; Yang, Y.; Peng, Z., Amorphous Li2O2: Chemical Synthesis and Electrochemical Properties. Angewandte Chemie 2016, 55, 10717-10721. 23. Nie, P.; Liu, X.; Fu, R.; Wu, Y.; Jiang, J.; Dou, H.; Zhang, X., Mesoporous Silicon Anodes by Using Polybenzimidazole Derived Pyrrolic N-Enriched Carbon toward High-Energy Li-Ion Batteries. ACS Energy Lett. 2017, 2, 1279-1287. 24. Bao, W.; Liu, L.; Wang, C.; Choi, S.; Wang, D.; Wang, G., Facile Synthesis of Crumpled Nitrogen-Doped MXene Nanosheets as a New Sulfur Host for Lithium-Sulfur Batteries. Adv. Energy Mater. 2018, 8, 1702485. 25. Shelke, M. V.; Gullapalli, H.; Kalaga, K.; Rodrigues, M. F.; Devarapalli, R. R.; Vajtai, R.; Ajayan, P. M., Facile Synthesis of 3D Anode Assembly with Si Nanoparticles Sealed in Highly Pure Few Layer Graphene Deposited on Porous Current Collector for Long Life Li-Ion Battery. Adv. Mater. Interfaces 2017, 4, 1601043. 26. Wang, J.; Lv, C.; Zhang, Y.; Deng, L.; Peng, Z., Polyphenylene Wrapped Sulfur/Multi-Walled Carbon Nano-Tubes via Spontaneous Grafting of Diazonium Salt for Improved Electrochemical Performance of Lithium-Sulfur Battery. Electrochim. Acta 2015, 165, 136-141. 27. Geng, X.; Zhang, Y.; Han, Y.; Li, J.; Yang, L.; Benamara, M.; Chen, L.; Zhu, H., Two-Dimensional Water-Coupled Metallic MoS2 with Nanochannels for Ultrafast Supercapacitors. Nano Lett. 2017, 17, 1825-1832. 28. Zhang, Y.; Cairns, E. J.; Ji, L.; Rao, M., Graphene Oxide as a Sulfur Immobilizer in High Performance Lithium/Sulfur Cells. J. Am. Chem. Soc. 2011, 133, 18522-18525. 29. Lin, M. H.; Hy, S.; Chen, C. Y.; Cheng, J. H.; Rick, J.; Pu, N. W.; Su, W. N.; Lee, Y. C.; Hwang, B. J., Resilient Yolk-Shell Silicon-Reduced Graphene Oxide/Amorphous Carbon Anode Material from a Synergistic Dual-Coating Process for Lithium-Ion Batteries. Chemelectrochem 2016, 3, 1446-1454. 30. J, L.; X, T.; J, Z.; Y, X.; H, H.; L, Z.; Y, G.; C, L.; W, Z., Sn 4+ Ions Decorated Highly Conductive Ti3C2 MXene: Promising Lithium-Ion Anodes with Enhanced Volumetric Capacity and Cyclic Performance. ACS Nano 2016, 10, 2491-2499.


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31. Salitra, G.; Markevich, E.; Afri, M.; Talyosef, Y.; Hartmann, P.; Kulisch, J.; Sun, Y. K.; Aurbach, D., High-Performance Cells Containing Lithium Metal Anodes, LiNi0.6Co0.2Mn0.2O2 (NCM622) Cathodes, and Fluoroethylene Carbonate-Based Electrolyte Solution with Practical Loading. ACS Appl. Mater. Interfaces 2018, 10, 19773-19782.


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Figures

a

Magnesium

TEOS

PMMA

650 ℃ MXene

MXene/SiOx

MXene/Si

Carbonization MXene/Si@SiOx@C

c

b

300 nm

500 nm e

d

500 nm g

f

C-Si (111) 10 x d ≈ 3.1 nm

500 nm

2 nm

10 nm

h 500 nm

Si

Ti

C

O

N

Figure 1. (a) Schematic preparation procedure of MXene/Si@SiOx@C nanohybrids. SEM images of (b) MXene, (c) MXene/Si and (d) MXene/Si@SiOx@C-2. (e) TEM and (f, g) HRTEM images of MXene/Si@SiOx@C-2. (h) Elemental mapping image of MXene/Si@SiOx@C-2.


MXene

Si/C D

30

45

e

Si 2p

105

2+

Si

8

9

10

1200 1500 1800-1 Raman shift (cm )

f

N 1s

MXene/Si@SiOx@C-2 MXene/Si@SiOx@C-3

MXene

600

300

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

g

Ti 2p

Graphitic N

Pyrrolic N Pyridinic N Ti-N

Intensity ( a.u.)

3+

Si

7

G

MXene/Si@SiOx@C-1

0

4+

Si

6

2Theta (degree)

Intensity ( a.u.)

Si

5

900 -1

MXene Si

Intensity (a.u.)

MXene/Si@SiOx@C-3

Intensity (a.u.)

MXene/Si@SiOx@C-3

Si (220)

MXene/Si@SiOx@C-3

MXene

d


2Theta (degree)

Si

c


Si (111)

15

b

MXene/Si@SiOx@C-2

3

MXene/Si@SiOx@C-1 MXene (002)

Intensity (a.u.)

a

Intensity ( a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Voume (cm g )

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Ti-C

Ti-N

+

102

99

Binding energy (eV)

404

400

396

Binding energy (eV)

464

460

456

Binding energy (eV)

Figure 2. characterization of MXene/Si@SiOx@C. (a, b) XRD patterns, (c) Raman spectra and (d) N2 adsorption/desorption isotherms of MXene/Si@SiOx@C-1, MXene/Si@SiOx@C-2 and MXene/Si@SiOx@C-3. High-resolution (e) Si 2p, (f) N 1s and (g) Ti 2p XPS spectrum of MXene/Si@SiOx@C-2.


ACS Nano

MXene/Si@SiOx@C-2

b 150

c

MXene/Si@SiOx@C-1

Commercial Si/C MXene

2.5


1

100

-Z" (ohm)

0

-1

2.0

Commercial Si/C

1.0

1.5

2.0

Voltage (V)

0.0

0

2.5

0

50

100

100 MXene/Si@SiOx@C-1

MXene/Si@SiOx@C-3

MXene/Si@SiOx@C-2

Commercial Si/C

80

1600 60 40 20 0 0

40

80

120

0 200

160

600

2.0

5C 2C

1 C 0.5 C

0.5 C

0.5 C

1400 1C

MXene/Si@SiOx@C-2 2C

700

5C 10 C

Si

0 0

30

60

0.5 0.0 500

1000

1500

120

100 75

1000

1.0

Specific capacity (mAh g-1)

90

Cycle number

1500

1.5

0

3000

2100

-1

10 C

2400

2000

Specific capacity (mAh g )

g

1800

2800

Cycle number

2.5

1200


e Coulombic efficiency (%)

2400

800

0

150

Z' (ohm)


0.5

120


st

1

0.5

0.0

f

th

100

1.0

50

-2

d

th

200

1.5

2000

50

500

25

0 0

200

400

600

800

0 1000

Coulombic efficiency(%)

Current (mA)

2

Voltage (V)

a

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Cycle number

Figure 3. Electrochemical performance in half cell. (a) CV curves of MXene/Si@SiOx@C-2, commercial Si/C and MXene electrode in the first cycle at a scan rate of 0.1 mV s -1. (b) EIS of MXene/Si@SiOx@C-1, MXene/Si@SiOx@C-2, MXene/Si@SiOx@C-3 and commercial Si/C. (c) Charging/discharging profiles of MXene/Si@SiOx@C-2 at 0.2 C at different cycles. (d) The cycling performance of MXene/Si@SiOx@C-1, MXene/Si@SiOx@C-2, MXene/Si@SiOx@C-3 and bare Si electrodes at 0.2 C for 200 cycles. (e) The charge/discharge profiles of MXene/Si@SiOx@C-2 electrode at different current rates. (f) Comparison of rate capabilities of MXene/Si@SiOx@C-2 with various rates from 0.5 to 10 C. (g) The cycling performance of MXene/Si@SiO x@C-2 in 1000 cycles at 10 C, (1 C = 4200 mA g-1).


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b

a

After 1000 cycles

c

After 1000 cycles

2.5 V

1.5 V

1.0 V

Charge

2.0 V

50 μm

300 μm d

Intensity (a.u.)

Before cycle

e

After 1000 cycles

0.5 V

0.01 V

28 um

25 um

0.5 V

50 μm

50 μm 1.0 V

1.5 V

2.0 V

Discharge

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

f

After 1000 cycles

g

After 1000 cycles

2.5 V

5

6

7

8

9

2Theta (degree)

10

500 nm

500 nm

Figure 4 Characterization of MXene/Si@SiOx@C-2 electrodes. (a) Ex-situ XRD patterns of the MXene/Si@SiOx@C-2 electrodes in the discharge-charge process. SEM top-view images of (b) MXene/Si@SiOx@C-2 (Inset: corresponding digital photographs) and (c) bare Si electrodes (Inset: corresponding digital photographs) after 1000 cycles at 10 C. Cross-sectional SEM images of (d) pristine and (e) cycled MXene/Si@SiOx@C-2 after 1000 cycles at 10C. (f) TEM and (g) SEM images of cycled MXene/Si@SiOx@C-2 particle after 1000 cycles at 10 C.


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a

c

Voltage (V)

b 4.5

0 min

4.0

10 min 60 min

st

3.5

1

th

20

Full Cell

th

50

th

100

3.0

120min

th

150

th

200

50

100

150

200

250


300

150 min

d

100 200 75 150

90o

0o

180o

0o

100

50 25

50 0 0

50

100

150

0 200

Coulombic efficiency (%)

0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Cycle number

Figure 5 Electrochemical performance in full-cell. (a) Schematic of the configuration of an MXene/Si@SiOx@C//Li[Ni0.6Co0.2Mn0.2]O2 full battery. (b) Charge/discharge curves of Al-plastic film soft package Li-ion full battery (inset) with MXene/Si@SiOx@C-2//Li[Ni0.6Co0.2Mn0.2]O2 at 0.2 C. (c) Photograph of an LED powered by a soft package Li-ion full battery for 150 min. (d) Cycling performance of soft package Li-ion full battery at 0.2 C under bending. (1 C = 280 mA g-1).


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Table 1 Mass percentage1 of MXene/Si@SiOx@C samples. XPS samples

Si mass wt%

SiOx mass wt%

MXene wt%

mass C mass N mass wt% wt%

MXene/Si@SiOx@C-1

64.8

2.3

25.6

6.2

1.1

MXene/Si@SiOx@C-2

72.8

2.5

16.9

6.5

1.3

MXene/Si@SiOx@C-3

78.7

3.0

10.7

6.4

1.2

1

Mass percent was calculated by the atomic ratio of Si, O, Ti, C and N in XPS survive scan.


ACS Nano

TOC Entry The MXene/Si@SiOx@C layer-by-layer superstructure with auto-adjustable layer space for boosting lithium-ion batteries was demonstrated. Rationally designed layer-by-layer superstructure exhibits the merits of increased active surface area, favorable electronic conductivity, rapid Li + mobility, flexible layer spacing and rich covalent bonds, which are ideal materials for accommodating the large volume changes of Si during cycling.

TOC Figure e-

eDischarge

e-

e-

Al foil

Li+

Cu foil

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Li+

e-

eMXene/Si@SiOx@C NCM-622 Charge -

e

e


Si@SiOx@C Layer-by-Layer Superstructure with

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